The field of the invention is centralizers that are used in conjunction with tubulars that are to be expanded downhole.
Centralizers have been in use for a long time. As their name suggests they have been used to center a tubular in a borehole. Most commonly, centralizers are used to position casing in the borehole as the cement is delivered into the annular space around the outside of the casing to set up and seal the casing in the bore hole. Centralizers have also been used as guides for sucker rods in downhole pumps. The centralizers have been made of metal and non-metallic materials such as thermoplastic polyamides, glass and mineral filled nylons and poly-tetra fluoro ethylene, also known as Teflon and injection molded polyurethane. These centralizers were made in hinged segments that could be clamped onto a tubular and in some applications the centralizers were formed right on to or slipped over the rod or tubular. Illustrative examples of the variations in prior centralizers are shown in U.S. Pat. Nos. 4,483,395; 4,088,185; 3,963,075; 2,611,664; 5,908,072; 6,102,118; and 6,283,205.
More recently, tubulars such as casing have been expanded downhole after cementing and sometimes without cementing. The centralizers used on casing for expansion have been metallic and have caused problems. Recently, one such problem has been reported in the March 2002 issue of Drilling Magazine on page 36. There a tempered steel arm of a centralizer had broken off and damaged the pipe to be expanded. The problem is that if the pipe to be expanded is scored prior to expansion, the stress is concentrated at that area and a fracture is likely upon expansion. There has been some recognition of this concern in the way the expandable tubulars are handled on the surface. Expandable tubulars are picked up with forklifts that have padded forks. The joints are packaged with wooden dividers to avoid contact with each other. Nylon slings are used to pick up joints one at a time onto the rig. Non-penetrating tongs are used to get a friction grip on the connections during makeup. Despite all these surface handling precautions, metallic centralizers have continued to be used. These centralizers are sufficiently rigid to increase the force required on the swage for expansion downhole. At times, the swage has stalled as the stroker has tried to advance it in the location of a relatively unyielding centralizer. Occasionally, the metallic centralizers used on casing to be expanded have had flexible strips break during run in and have scored the outer surface of the casing to the point that when the casing was expanded the stress concentration at the point of scoring initiated a fracture failure during expansion.
The object of the present invention is to provide centralizers for tubulars that are to be expanded that are compatible with downhole environmental conditions and are flexible enough so as to avoid significantly increasing the force required to expand the pipe and centralizers combination. Additionally, the centralizers are preferably non-metallic to avoid scoring the pipe during run in or if a piece of the centralizer should break leaving an exposed end. The soft nature of the preferred centralizer, allows it to act as a seal for the cement at one or both ends, as the expansion pressure reforms the ring shaped ends of the centralizer to assume the shape of the borehole wall to minimize channeling along the outside of the tubular even though there has been cement placed in the annulus around the tubular. These and other advantages of the present invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the claims below.
Centralizers made of a non-metallic material in a variety of styles are placed on the exterior of tubing or casing prior to expansion. The compliant nature of such materials, when put into service in a centralizer for tubulars to be expanded prevents scoring the pipe on the way downhole. Scratches or scores of the pipe can be the location of stress fractures on expansion. Additionally, the resilient nature of the centralizers prevents them from adding considerably to the required expansion force and allows them to act as seals against channeling in the cemented annular space after expansion.